In the manufacture of straight-chromium ferritic stainless steels such as AISI Types Nos. 430,434 and 436, it is a preferable practice after hot rolling to produce hot-band coils welded together at the coil ends to form a larger coil for use during subsequent cold rolling. The larger coil so-formed is desirable for large-capacity cold-rolling facilities. The preferred practice for joining hot-band coils is by autogenous welding, wherein no weld filler metal is used. This practice provides a chemical composition at the weld area that is the same as the remainder of the coil. It is not, therefore, necessary to locate and remove the weld portion upon completion of cold rolling. In large-capacity cold-rolling operations, larger hot-band coils of this type serve to reduce handling and processing costs.
It has been found, however, that with straight-chromium ferritic stainless steels upon autogenous welding of two coils during subsequent cold rolling, the weld area is brittle and thus this weld area of the coil cannot be passed over processing rolls incident to cold rolling. In addition, during cold rolling to finished gauge, the coils tend to break at the weld area. The brittleness of the weld area results from the formation of martensite, which is a hard and brittle phase, upon cooling from the elevated welding temperatures. To overcome this problem, it is known to subject the weld area of the coil to a tempering treatment to soften the brittle martensite much as in a similar manner that the coil was final or box annealed. Although tempering treatments sufficiently lower the ductile-brittle transition temperature (DBTT) for some lighter gauge strip materials below the conventional hot-band thickness, they are not sufficient for typical hot-band gauges. Even if the martensite is softened by tempering, the coils will, nevertheless, tend to break during cold rolling if the ductile-brittle transition temperature is at or above the average room temperature of about 70.degree. to 75.degree. F.
It is accordingly a primary object of the present invention to provide a post-weld treatment for autogenous welded hot-band straight-chromium ferritic stainless steel that will soften the martensite of the weld area and, in addition, improve the ductility and toughness so that cracking of the weld area is avoided during subsequent cold rolling to final gauge.
This and other objects of the invention as well as a more complete understanding thereof may be obtained from the following description and specific examples.